Low Poly Mountain Terrain
Here is what I got:





Analysis:

According to the result, there are some evidences that the author might very possibly take AO pass into account:



So the basic idea is to multiply AO pass with Image pass something like this:



Then it should come with a few color related nodes such as Color Correction, Color Balance, RGB Curves, etc. to mimic the tones for each channel, something like below:



Materials are very basic, no magics I bet. Just use Sun lamp with a very small size value to get sharp shadow:



There are generally two methods to model poly mountains:

Method A:


Create a plane, then subdivide and decimate:

Apply all modifiers, then paint vertex weights in Weight Paint mode, and use it for a Displace modifier:



Method B:

Use Sculpting tools. Use dynamic topology to create these randomized triangles, and use Flatten brush to create the flat tops: